Request E-Signature link does not work Acrobat Pro Desktop Version

I can create signature fields, however when I email them to be signed and the signer sends it back, it indicates that their Digital ID doesn't have a valid certificate (gives an error that at least one signature has problems, see attachment).

Have you checked why your Acrobat warns about the returned signatures?

Have the signed simply created self-signed certificates and used them for signing? (In that case there obviously is no reason for your Acrobat to trust them by default but you could decide to trust those certificates manually.)

Or have they used valid certificates issued by a PKI from the Adobe or European trust list? (In that case the list of trusted identities in your Acrobat might be outdated or broken. Consider updating or repairing. )